The wireless telecommunications market has experienced an enormous subscriber growth in recent years, with no signs of slowing. It is therefore becoming more and more essential for wireless network operators to be able to increase capacity (i.e. support more subscribers) with the limited available spectrum of transmission frequencies.
There are currently a number of techniques in use or under development to improve the downlink (base-station-to-mobile subscriber) transmission capacity of wireless networks. The most common technique is to use sectored antennas within cells whereby either separate antennas or separate beams of a phased array antenna system are used to cover different geographic sectors within the cell. Other techniques for increasing network capacity include time division multiple access (“TDMA”) and frequency division multiple access (“FDMA”) such as used by, for example, the GSM (“Global System for Mobile telecommunications”) system predominant in Europe. Code division multiple access (“CDMA”) is an increasingly pervasive technique for expanding subscriber capacity.
Most typically, cells are divided into three or six sectors, where each sector covers 120° or 60° degrees respectively. With this approach, the capacity of a cell with three equal-sectored (120°) antennas, for example, is increased due to an increase in the reuse of channels throughout the network and/or improved signal quality associated with the use of directional antennas. For example, in a code division multiple access system, theoretically three times the capacity of that of a cell using an omnidirectional antenna at the base station may be achieved using a three sectored system. Ideally, the capacity could be improved n-fold by employing n 2p/n radian beamwidth antennas or antenna sectors. As a practical matter, this ideal increase is not possible due to the necessity and desirability of some beam overlap (see, Viterbi, CDMA, Principles of Spread Spectrum Communications, p. 227 (1995) incorporated herein by reference).
Yet another approach to increased wireless system capacity is the use of multiple beam, or “smart” antennas, such as shown and described in the above referenced patent application entitled “Polarization and Angular Diversity Among Antenna Beams.” Systems utilizing smart antenna systems have the ability to form or select relatively narrow beams for transmission depending upon a subscriber's position relative to the base station, rather than transmitting on a single beam encompassing the antenna's entire coverage area. The use of such multiple beam antennas provides for selection of relatively narrow antenna beams for providing communications with respect to a particular mobile unit. Accordingly, the amount of interfering energy experienced in the communicated signals is reduced because power is not wasted by having it transmitted to areas where it cannot be received by the subscriber.
Such smart antenna systems may be used in combination with standard frequency hopping techniques (see, for example, Kronestedt and Andersson, “Adaptive Antennas in Frequency Hopping GSM”, ICUPU 1998, pp. 325–29 incorporated herein by reference). Frequency hopping consists of altering the carrier frequency transmitted to a given subscriber between transmission time frames.
However, it should be appreciated that in the previously proposed schemes, the antennas are not truly adaptive. Adaptive antennas can be controlled to transmit relatively narrow, focused beams directed at an individual subscriber, where the beam width is chosen to be as small as possible to provide the desired or necessary power to the subscriber through the use of complex antenna signal weighting. Instead of providing such adaptive antenna configurations, the previously proposed schemes transmit on a series of fixed beams, such as may be associated with a fixed beam forming network, e.g., a Butler matrix.
Accordingly, a need exists in the art for systems and methods which provide increased subscriber capacity in a wireless communication system over that available through the use of currently available techniques, such as sectorization, switched beam smart antenna systems, and frequency hopping.